1. Field of the Invention
The present invention is generally directed to a photovoltaic receiver assembly for use in a photovoltaic concentrator module and a novel process for its construction. More specifically, the present invention is directed to a photovoltaic receiver comprising a plurality of photovoltaic cell packages, where each package includes a cell, two or more electrical conductors, one or more bypass diodes, and a light concentrating prism cover, where the cell packages are mounted to a heat sink and are encapsulated within an impermeable dielectric film to prevent the ingress of moisture.
2. Description of the Prior Art
Photovoltaic cells, commonly known as solar cells, are a well known means for producing electrical current from electromagnetic radiation. Traditional solar cells comprise junction diodes fabricated from appropriately doped semiconductor materials. Such devices are typically fabricated as thin, flat wafers with the junction formed parallel to and near one of the flat surfaces. Photovoltaic cells are intended to be illuminated through their so called "front" surface. Electromagnetic radiation absorbed by the semiconductor produces electron-hole pairs in the semiconductor. These electron-hole pairs may be separated by the junction's electric field, thereby producing a photocurrent.
Photovoltaic cells are presently used in two types of commercial power modules, namely flat-plate, one-sun modules, and concentrating, multi-sun modules. The former are typically mounted on roof tops or fixed flames. The latter typically track the sun's apparent motion and use lenses or mirrors to focus sunlight onto the photovoltaic cells. The primary advantage of the concentrating module is a substantial savings in required semi-conductor area. For example, a Fresnel lens can be used to focus sunlight at 20 times the normal one-sun intensity. To produce a fixed amount of power of, for example, 1 kW, a 20-sun concentrating module will use an amount of photovoltaic cell material which is 95% less expensive than a comparable flat-plate, one-sun module.
In concentrating modules, groups of photovoltaic cell packages are electrically coupled to form photovoltaic receivers. In addition to including a plurality of photovoltaic cells, contemporary photovoltaic receivers have included means to concentrate electromagnetic radiation onto the active surface area of each solar cell. One novel means to concentrate solar radiation in this fashion is disclosed in U.S. Pat. No. 4,711,972 issued to O'Neill, the disclosure of which is incorporated herein.
Disadvantages of traditional photovoltaic receivers have heretofore resided in the labor costs associated with their assembly. The photovoltaic receiver generally includes a heat sink, one or more solar cells, a light concentration means, bypass diode circuitry, electrical interfaces, and a dielectric encapsulant. One of the main difficulties in receiver construction is to satisfactorily encapsulate the cells so as to avoid electrical shorts, characterized by leakage currents, when the receiver is exposed to electrolyte-laden water. While it is possible to encapsulate solar cells to avoid such shorts, the assembly cost associated with such encapsulation has heretofore been prohibitive.
Other disadvantages have resided in the detrimental filtering effect traditional encapsulants have had on the solar cells. When traditional encapsulants and the means by which they are secured to the solar cell begin to break down due to effects of moisture and dirt, temperature extremes or the like, they frequently tend to discolor and thereby impede receipt of electromagnetic radiation by the solar cells. Such degradation diminishes the effectiveness of the photovoltaic receiver.
Still other disadvantages of traditional photovoltaic receivers relate to their manufacturing processes. Most, if not all, prior art receivers have been assembled using messy, expensive, and trouble-prone liquid adhesives to bond the solar cells to the heat sink, and still more messy, expensive, and trouble-prone liquid encapsulants to protect the receiver from the environment. Traditionally, these adhesives have been based on silicone materials, which require long time periods or elevated temperatures to cure, and which remain sticky and soft after curing. These properties are not desirable for long-term durability and longevity.
Further disadvantages of traditional photovoltaic receivers relate to their electrical and thermal properties. Traditional photovoltaic receivers incorporate photovoltaic cells which must be thermally coupled with the heat sink to maintain low operating temperatures and corresponding high electrical conversion efficiencies. However, the cells must be electrically isolated from the heat sink for reasons of personal safety. These thermal and electrical requirements have proven to be very difficult to achieve with reasonably priced materials.